Code Division Multiple Access (CDMA) is a method for multiple users to share common bandwidth over a communications channel. Instead of dividing the frequency spectrum of the channel into discrete sub-channels (frequency division multiplexing—FDM) or alternating use of a channel over time (time division multiplexing—TDM), CDMA mixes each user's transmission with a spreading code, typically a pseudo-random sequence having a starting point and a progression known by both the transmitter and the receiver. In this fashion, many users can share the same bandwidth at the same time. CDMA is one of methods specified for use in third-generation (3G) mobile communications by the International Telecommunication Union,(see, e.g., “The CDMA 2000 Candidate Submission”, Telecommunications Industry Association TIA TR-45.5 Subcommittee, June 1998). However, systems employing CDMA are typically interference-limited. The interference can arise from the very nature of the systems in which CDMA is used, e.g., cellular telephone, in which it is typically desirable to accommodate multiple users transmitting simultaneously through a common physical channel.
In telecommunications, “multipath” is a propagation phenomenon wherein a signal reaches a receiver by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from objects such as mountains and buildings. The effects of multipath include constructive and destructive interference, and phase shifting of the signal. Interference between signals, e.g., multiple access interference (MAI) can also arise from multiple users sharing a common bandwidth for different services, such as voice and data transmission. Data rate mismatches between users can result in code mismatch. Even in the absence of multipath, this can cause MAI.
Not surprisingly, since CDMA is currently-specified as a multiple-access scheme for 3G cellular telephone systems and methods mitigating the effects of multipath and providing MAI suppression in CDMA environments are highly desirable. Those systems and methods that are less computation-intensive than others are more desirable.
Typically, a multiple-delay-line filter, called a “rake,” is used to coherently combine multipath signals at the receiver. The phase and gain information used to cohere a rake filter typically is provided by a separate equalization function. Rake receivers, while designed to operate in multipath environments, are generally limited in performance by MAI. MAI causes the performance of a conventional rake receiver to degrade rapidly as the number of users are increased in a CDMA system. On the other hand, research in interference (e.g., MAI) suppression has typically focused on data-aided or blind Minimum Mean Square Error (MMSE) receivers, which are sub-optimal in multipath channels.